Game bird is all at sea (9)
I believe the answer is:
billiards
'game' is the definition.
(billiards is a kind of game)
'bird is all at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'bird'+'is'+'all'='birdisall'
'birdisall' with letters rearranged gives 'BILLIARDS'.
(Other definitions for billiards that I've seen before include "Game with three balls" , "Indoor sport" , "Game played with balls on table with pockets" , "Ball game" , "Table game at which Mary Queen of Scots excelled" .)